1. Field of the Invention
The present invention relates to a power transfer switch which changes over a power source from a commercial power to another power such as an emergency power when a power failure occurs in the commercial power that is connected to a load, and is used for automatically changing over the power source into the commercial power in a power return of the commercial power.
2. Description of Related Art
A power transfer switch is an apparatus for changing over a plurality of power sources to supply an electricity to a load, and is widely used in home or facilities provided with an emergency power (a preliminary power source) such as a power generator for taking a countermeasure against the power failure. In the change-over of the power source, a cross bar is rotated to cause a movable contact held on the cross bar to come into contact with a fixed contact on the opposite side, thereby changing over the power source.
It is desirable to change over the power source in a time as short as possible. In addition, it is necessary to reliably cause the movable contact to come into contact with the fixed contact after the change-over. As described in Patent Document 1, therefore, there is employed a structure in which a cross bar is rotated instantaneously by utilizing a toggle mechanism in the change-over of the power source, and furthermore, a spring is provided between the crossbar and the movable contact to maintain a contact pressure between the movable contact and the fixed contact by a spring force.
FIGS. 1 and 2 are views for explaining an operation of the conventional movable contact described in the Patent Document 1. A movable contact 51 is a metal plate obtained by bending a base portion 52, and the base portion 52 is flexibly supported on a cross bar 53 to be rotated by means of a toggle mechanism which is not shown. Moreover, a protruded piece 54 having an L-shaped cross-section is erected on the cross bar 53 with a clearance between the protruded piece 54 and the movable contact 51. Furthermore, a compression spring 55 is provided on the opposite side to the protruded piece 54 of the cross bar 53, and a lower part of the movable contact 51 is elastically pressed against a side wall of the protruded piece 54.
When the cross bar 53 is rotated in a direction of the protruded piece 54 (a leftward direction in the drawing) as shown in FIG. 1, a contact point on a tip of the movable contact 51 first comes into contact with a contact point of a fixed contact 56 on a left side. However, the cross bar 53 is further rotated excessively by a certain angle. Therefore, the movable contact 51 is pressed by a lower part of the side wall of the protruded piece 54. At this time, the compression spring 55 provided between the movable contact 51 and the cross bar 53 presses the lower part of the movable contact 51 toward the side wall of the protruded piece 54. Therefore, a rotation moment in a counterclockwise direction with a contact point A of the lower part of the side wall set to be a fulcrum is generated in the movable contact 51. For this reason, the contact point on the tip of the movable contact 51 is pressed in the direction of the fixed contact 56 to maintain a contact pressure between the contact points.
When the cross bar 53 is rotated in the opposite direction to the protruded piece 54 (a rightward direction in the drawing) as shown in FIG. 2, moreover, the contact point on the tip of the movable contact 51 first comes into contact with a contact point of a fixed contact 57 on a right side. However, the cross bar 53 is further rotated excessively by a certain angle. Therefore, the movable contact 51 is pressed by an upper part of the side wall of the protruded piece 54. At this time, the compression spring 55 provided between the movable contact 51 and the cross bar 53 presses the lower part of the movable contact 51 toward the side wall of the protruded piece 54. Consequently, a rotation moment in a clockwise direction setting a contact point P with an upper part of the side wall as a fulcrum is generated in the movable contact 51. For this reason, the contact point on the tip of the movable contact 51 is pressed in the direction of the fixed contact 57 to maintain a contact pressure between the contact points.
As described above, the power transfer switch described in the Patent Document 1 has the structure in which the contact pressure between the fixed contacts 56 and 57 on both of left and right sides and the movable contact 51 is generated by means of the single compression spring 55. In the state of FIG. 1, however, the fulcrum A serves as the lowest end of the movable contact 51, a power point B serves as the pressing point of the compression spring 55, and an action point C serves as the contact point on the tip of the movable contact 51. Therefore, a lever ratio of AB/AC is obtained. On the other hand, in the state of FIG. 2, the fulcrum P is positioned between the power point B and the action point C. Therefore, a lever ratio of PB/PC is obtained. Thus, the lever ratios are not equal to each other in the states of FIGS. 1 and 2. For this reason, there is a problem in that the contact pressures to be applied to the contact points by the compression spring 55 are not equal to each other.